When a fuel gas is combusted using a burner, prior to the fuel gas being fed to the governor, it is mixed with air and is fed to the governor as a mixed gas of the fuel gas and the air. The control of the air fuel ratio (A/F) for this mixed gas is indispensable in optimizing the mixed gas, or in other words, in optimizing the state of combustion of the fuel gas (to ensure the full combustion thereof).
This A/F ratio maintains the air/fuel ratio A/F at the uniform and ideal air/fuel ratio by measuring the fuel gas provision rate and the air provision rate (the mass flow) for the mixed gas, and adjusting the gas provision rate and the air provision rate based on the results of the measurement. (See, for example, Japanese Unexamined Patent Application Publication 2002-267159.) Thermal mass flow gauges, for example, may be used in the measurements of the amount of gas and air supplied.
On the other hand, when producing the mixed gas there are cases wherein various types of fuel gases having different compositions are used, or wherein there are differences in the composition even when the same type of fuel gas is used. In order to perform the A/F control under such circumstances, the calorific value of combustion in the fuel gas used or the calorific value per unit time is calculated and the calorific value of combustion or calorific value is fed back to the A/F control. (See, for example, Japanese Unexamined Patent Application Publication 2003-35612.)
Furthermore, in addition to air, oxygen may also be used when producing the mixed gas, and, in such a case, the mass flows of the fuel gas, the air, and the oxygen are each measured separately for the A/F control and the O2/F control (abbreviated here as oxygen/fuel ratio control). (See, for example, Japanese Unexamined Patent Application Publication 2007-87029 (“JP '029”).)
Note that when the burner uses a glass tube sealed process, high precision control is required for the amount of calorific value of the mixed gas, that is, of the fuel. In other words, while on the one hand the amount of fuel gas supplied is controlled based on the mass flow of the fuel gas, measured by a thermal mass flow gauge, as described above, on the other hand the amounts of air and/or oxygen supplied relative to the amount of fuel gas supplied is controlled so as to have the respective ideal mixtures of fuel gas, air, and/or oxygen in the mixed gas.
However, even when control is performed in this way, when there is a change in the composition of the fuel gas, then rather than maintaining the calorific value of the mixed gas that includes the fuel gas at a desired control value, or rather than maintaining the calorific value of combustion or calorific value per unit time at a desired control value, conversely there is the danger that the air and/or oxygen mixing ratio relative to the fuel gas will vary due to the density of the fuel gas within the mixed gas varying as well, resulting in the danger of incomplete combustion of the fuel gas.
The object of the present invention is to provide a flow rate controlling device capable of controlling the flow of a fuel gas based on the calorific value by the fuel gas to stabilize, for example, a fuel gas at a constant calorific value, notwithstanding differences or changes in the composition of the fuel gas.